First Prior Art
In a first conventional analog type PWM circuit, a PWM signal is obtained by comparing an isosceles triangle reference signal and a signal to be modulated (referred to as a modulation signal, hereinafter) with a comparator. In this case there is no phase change of the PWM signal dependent on the magnitude of the modulation signal.
On the other hand, there is a method for realizing a digital type PWM circuit. As shown in FIG. 2A, the sawtooth waveform C in the form of a right-angled triangle generated by a ring counter is used as a reference signal, and this reference signal and modulation signal D are input to a magnitude comparator, in which the magnitudes of both signals are compared with each other. The comparison result of the magnitude comparator is used as a PWM signal (PWM 1). This digital type method is easily obtained from the analog type method. This digital type method has the excellent advantage that it requires only a magnitude comparator when multi-channel PWM is realized. This method also has the feature that if the cycle to update modulation data is equal to that of the ring counter or any integral multiple of the ring counter cycle, the modulation characteristic will become ideally linear and therefore the resolution and carrier cycle of the PWM signal obtained will be equal to those of the ring counter.